Cells in the nucleus raphe' magnus of the rat's medulla, which are known to modulate withdrawal reflexes from painful stimuli, will be recorded with extracellular microelectrodes while experimentally excited nearby with electrical current or glutamate ions, and at times the tail will concurrently receive noxious thermal stimulation. Attention will first be given to the physical aspects of these two forms of experimental excitation, particularly as regards the spatial extent of their actions and their differential effect on cell bodies and axons. Profiting from the resulting precision in specifying how many neurons of each kind are excited, the final aim is to gauge the degree to which individual off-cells and on-cells, which are respectively inhibitors and facilitators of flexion, influence the occurrence of the tail- flick reflex. The importance of relative timing among impulses in mixed groups of neurons will also be studied. This work will not only better characterize a descending inhibitory system that is crucial in opiate analgesia, it will provide basic data on electrical brain stimulation which may improve the efficiency of the technique whether for surgical relief of chronic intractable pain or for any other scientific or medical application.